This invention relates generally to a method and apparatus for estimating a frequency offset. More particularly, this invention relates to a method and apparatus for estimating the frequency offset between a carrier frequency of a transmitter and a local frequency reference of a receiver in a communication system.
In any communication system, it is important for a receiver to be synchronized with a transmitter so that messages can be successfully exchanged between the transmitter and the receiver. In a radio communication system, in particular, it is important that a receiver be tuned to the frequency of the transmitter for optimal reception.
In a typical radio communication system, remote stations communicate with one or more base stations via a radio air interface. Various approaches have been employed to prevent transmissions between the various base stations and remote stations from interfering with each other.
In some radio communication systems, neighboring base stations are each assigned a different carrier frequency with which to communicate with remote stations so that transmissions from one base station do not interfere with transmissions from a neighboring base station. In addition, to prevent transmissions between each of the remote stations and a particular base station from interfering with each other, Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) have been employed. In radio communication systems using FDMA, each remote station is assigned a particular frequency with which to communicate with a base station. In systems using TDMA, a base station allocates a particular time slot or slots within a frame to each remote station. Some remote stations can use the same frequency but different time slots to communicate with the base station.
In other radio communication systems, the Code Division Multiple Access (CDMA) method has been employed. According to the CDMA method, each remote station is assigned a particular digital code word(s) that is orthogonal to code words assigned to other stations. Neighboring base stations can exchange messages with remote stations using the same frequency but different digital orthogonal code words to indicate which remote station the messages are designated for.
Whether a radio communication system employs FDMA, TDMA, CDMA, a combination of these approaches, or some other approach, it is important for a remote station to be time and frequency synchronized to the base station serving the area from which it desires to communicate. In other words, the local frequency reference of the remote station must be tuned to the carrier frequency of the base station, and the local time reference of the remote station must be synchronized to the time reference of the base station. A periodic synchronization signal is typically transmitted from the base station to the remote station for this purpose.
For initial synchronization in a system employing the European Global System for Mobile Communication (GSM) standard, the carrier frequency of the base station is modulated from time to time with a Frequency Correction Burst (FCB) and a Synchronization Burst (SB) to form a frequency synchronization signal. The carrier frequency of the base station is typically modulated with the FCB using Gaussian Minimum Shift Keying (GMSK). A typical FCB is a sequence of 148 symbols, each symbol a zero, that transforms into a pure sinusoidal signal after modulation. The frequency of the resulting frequency synchronization signal is thus equal to 1/4T Hz, where T represents a symbol duration of the sinusoidal signal, and there are four symbols per cycle of the sinusoidal signal. T is typically 48/13 microseconds (.mu.sec), so that the frequency synchronization signal has a frequency of approximately 67.7 KHz. The FCB is repeated every tenth frame for the first four times, and then for the fifth time, the FCB is repeated on the eleventh frame. This frame sequence is then repeated indefinitely, to maintain synchronization between the remote station and the base station.
From the information in the FCB, the remote station is able to roughly synchronize itself with the time slot(s) allocated to it. This rough time synchronization is then sufficient to locate the SB, which is typically located eight bursts after the FCB, and to decode the information it carries. The information obtained by decoding the SB is then used to finely tune the local frequency reference of the remote station to the carrier frequency of the base station and to adjust the remote station's local time reference to the time slot(s) allocated to it by the base station.
In systems employing CDMA, each base station transmits a frequency synchronization signal in the form of, for example, a pilot sequence on each of the frequencies assigned to that particular base station as well as possibly on some or all of the frequencies that are not assigned to that particular base station. If the frequency has been assigned to the base station, the corresponding pilot sequence may be transmitted with slightly more power than the other frequencies used by the base station. Each remote station receiving the carrier modulated by the pilot sequence demodulates the signal. As a result, each remote station can receive signals designated for it and simultaneously measure the signal strengths of neighboring base stations using different carriers. This information is used by the remote station to determine which received pilot sequence has the strongest signal strength, and the local frequency reference of the remote station is adjusted to the appropriate carrier frequency, accordingly.
Any frequency difference between the local frequency reference of the remote station and the carrier frequency of the base station is readily detected in the demodulated frequency synchronization signal. For example, in systems employing the GSM standard, the difference between the frequency of the modulated frequency synchronization signal, which is known to be 67.7 KHz, and the frequency of the received frequency synchronization signal, demodulated to the baseband, is a direct measure of the error in the local frequency reference of the remote station. In systems employing CDMA, the difference between the known frequency of the strongest transmitted pilot sequence and the frequency of the demodulated pilot sequence is used by the remote station as a measure of the error in the local frequency reference of the remote station.
A number of approaches have been introduced for estimating the frequency difference between the remote station's local frequency reference and the carrier frequency of the base station, taking into account phase variations that may occur in the transmitted frequency synchronization signal due to modulation. From this estimated frequency difference, the carrier frequency of the base station can be derived.
For example, U.S. Pat. No. 4,847,872 to Hespelt et al. discloses a method and arrangement for synchronizing receivers in digital transmission systems. A preamble pattern is transmitted and demodulated such that the received signal has a cosine shape. The frequency and/or phase of the carrier are estimated, and individual spectral lines of the received signal are obtained by filtering. The frequencies and phases associated with the spectral lines are determined by linear regression, and the carrier frequency offset is estimated based on these values.
U.S. Pat. No. 5,416,800 to Frank discloses a mobile radio receiver for a radio transmission system including a recognition circuit which detects pulses of a frequency synchronization signal including a FCB and derives a time position signal from the detected pulses using linear regression. The radio receiver also includes a frequency estimating circuit which estimates a frequency deviation based on the received signal. The time position signal and the frequency deviation signal are used to track the frequency of the carrier wave. According to this patent, the entire FCB is used to estimate the frequency deviation.
The approaches described in these two patents suffer from the same problem, namely, that the number of samples and the corresponding number of computations used to estimate the frequency offset consume a significant amount of memory. To simplify computation, these patents disclose "phase unwrapping", i.e., limiting the range of phase variation in the received signal to .+-..pi.. If the phase difference between successive samples is outside of the interval (-.pi., .pi.), the most recently collected sample is "unwrapped", i.e., phase shifted by -2.pi. or 2.pi., and this phase shift is tracked, e.g., stored in a memory in association with the corresponding sample. This consumes even more memory and results in a complex system.
Due to their large memory requirements, the approaches disclosed in these patents are typically implemented in software. This consumes large amounts of power. Since time synchronization must be performed when a remote station is in the idle stand-by mode, and remote stations are often battery powered, power consumption is an important consideration. The higher the power consumption, the lower the available stand-by time.
There is, thus, a need for a simple method for estimating a frequency offset which consumes a minimal amount of power and memory and which overcomes the drawbacks noted above.